A few soils having widely different reactions have been taken for finding the effect of soil-water ratio on pH value. It has been found that alkali soils generally show a maximum pH content at a dilution 1:2, whereas acid soils generally show a minimum between 1 : 0.5 to 1:2 dilution. These results have been explained on the hypothesis that soil colloidal complex acts as a salt of weak acid; dissociation and hydrolysis can occur depending upon the nature of the clay complex and exchangeable bases. The extrapolation of pH value at low moisture content from values obtained at higher dilution, is not feasible. It has been concluded that the pH value at saturation moisture should be determined in case of fertility studies as this value will more closely approximate the actual condition in the field. For soil classification purposes, however, the pH value at l : 5 dilution may be carried out.
